Semiconductor laser with a lattice structure

ABSTRACT

A semiconductor laser ( 22 ) with a semiconductor substrate ( 11 ), a laser layer ( 13 ) arranged on the semiconductor substrate, a waveguide ridge ( 15 ) arranged at a distance from the laser layer, and a strip-shaped lattice structure ( 23 ) arranged in parallel to the laser layer is disclosed. The lattice structure ( 23 ) includes two structural regions ( 24, 25 ) which are arranged on both sides of the waveguide ridge ( 15 ) and are formed at a distance from the laser layer ( 13 ) above the laser layer ( 13 ). A process for the production of such a semiconductor laser is also disclosed.

RELATED APPLICATIONS

This is a Divisional Application of application Ser. No. 09/296,059filed Apr. 21, 1999 now U.S. Pat. No. 6,671,306, and the entiredisclosure of this prior application is considered to be part of thedisclosure of the accompanying application and is hereby incorporated byreference therein.

FIELD OF THE INVENTION

The present invention relates to a semiconductor laser with asemiconductor substrate, a laser layer arranged on the semiconductorsubstrate, a waveguide ridge arranged at a distance from the laserlayer, and a strip-shaped lattice structure arranged in parallel to thelaser layer. The present invention further relates to a process for theproduction of such a semiconductor laser.

BACKGROUND OF THE INVENTION

Known semiconductor lasers of the type defined in the introduction, alsoreferred to in the art as so-called DFB (distributed feedback) laserdiodes, have a lattice structure which extends through the laser layerand which facilitates the construction of a monomode laser diode inwhich, in contrast to multi-mode laser diodes, laser radiation with onespecified laser mode is emitted and other modes are suppressed by thelattice structure. The production of the DFB laser diodes constructed inthe known manner proves extremely costly, in particular due to theproduction and test process employed and the high reject quotaassociated therewith. For the production of the known DFB laser diodeson the basis of a wafer on a semiconductor substrate base, epitaxy isused to form the structure of the semiconductor wafer on thesemiconductor substrate. For the formation of the lattice structure inthe laser layer, when approximately half the layer height of theepitaxial structure has been reached the epitaxial growth is interruptedand the lattice structure is introduced in a lithographic—and removalprocess. Then the epitaxial growth is continued. The interruption of theepitaxy in the formation of the laser layer and the following overgrowthof the lattice structure introduced into the half-layer induces defectsin the laser layer which disadvantageously affect the properties of thelaser layers and possibly manifest in a higher current consumption or areduced life of the laser diodes.

As a result of the mutual influences between the laser layer and thelattice structure formed in the laser layer in terms of theamplification properties of the semiconductor laser wafer, theproperties of a semiconductor laser wafer produced in the described waycannot be predetermined in an exact manner. As the properties of thesemiconductor laser wafer cannot be determined until after theconclusion of the epitaxial growth and the complete formation of thelaser layer in the test operation, the amplification spectrum of thesemiconductor laser wafer also cannot be determined until after theformation of the lattice structure in the laser layer, with the resultthat the lattice structure cannot be accurately adapted to theamplification spectrum of the laser layers and consequently the knownDFB laser diodes also cannot be produced in a precise manner inaccordance with predefined specifications relating to the desired lasermode or the desired wavelength. Rather, the structure of the known DFBlaser diodes described in the foregoing requires a production process inwhich different lattice structures must be formed in the laser layer ofa semiconductor laser wafer in order that, by checking the laser diodesseparated from the semiconductor laser wafer, precisely those laserdiodes which emit the desired laser mode with the desired wavelength canbe retrospectively determined. It is thus apparent that the structuraldesign of the known DFB laser diodes necessitates the production of aplurality of laser diodes in order that the laser diodes suitable forthe intended application, i.e. those laser diodes which emit a laserradiation with the desired wavelength, can be selected from thisplurality of laser diodes by testing of their laser properties.

SUMMARY AND OBJECTS OF THE INVENTION

The primary object of the present invention is to propose a laser diodewith a structure which facilitates a simple and reproducible manufactureof laser diodes with a defined wavelength. It is also an object of thepresent invention to propose a DFB laser diode with improved poweroutput. A further object of the present invention is to propose aprocess particularly suitable for the production of a DFB laser diodeaccording to the invention.

According to the invention, a semiconductor laser is provided with asemiconductor substrate. A laser layer is arranged on the semiconductorsubstrate. A waveguide ridge is arranged at a distance from the laserlayer, and a strip-shaped lattice or grating structure is arranged inparallel to the laser layer. The lattice structure includes twostructural regions which are arranged on both sides of the waveguideridge and are formed at a distance from the laser layer above the laserlayer.

An embodiment of the invention provides a DFB laser diode with a latticestructure produced following the conclusion of the epitaxial growth ofthe laser layer for completion of the semiconductor laser wafer andfollowing the formation of the waveguide ridge. By virtue of thisstructurally required, subsequent production of the lattice structure itis possible to determine the individual amplification spectrum of thelaser layer and semiconductor laser wafer before the production of thelattice structure in order then, by selective predefinitions of theparameters of the lattice structure, to be able to subsequently producethe desired laser profile in an exact manner and thus to be able toreproducibly manufacture DFB laser diodes with a precisely definedwavelength or laser mode.

The structural design according to the invention also facilitates anundisturbed, continuous formation of the laser layer in the epitaxialprocess so that unnecessary defects, which can impair the power outputcharacteristic of the laser layer or the DFB laser diode, do not ariseat all. The arrangement of the lattice structure at a distance from theactive laser layer also prevents the subsequent impairment of the laserlayer. The lattice structure modulates periodically the losses and therefractive power for light propagating through the laser. In this waythe DFB laser diode according to the invention facilitates a complexcoupling of the laser radiation with the lattice structure with lateralmodulation of the real—and imaginary parts of the refractive index.Laser diodes according to the invention therefore have a high degree ofinsensitivity to back-reflections, which enables them to be used withoutan optical isolator, for example in applications for glass fibertransmission.

To permit the precisest possible setting of the distance or relativeposition between the lattice structure and the active laser layer of theDFB laser diode in the production of the DFB laser diode, the latticestructure can be arranged on a barrier layer arranged in parallel to thelaser layer.

If a metal, for example chromium, aluminum, scandium, titanium,vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc,yttrium, zirconium, niobium, molybdenum, ruthenium, rhodium, palladium,silver, cadmium, tin, hafnium, tantalum, tungsten, rhenium, osmium,iridium, platinum, gold, thallium, lead, bismuth, lanthanum, cerium,praseodymium, neodymium, promethium, samarium, europium, gadolinium,terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium andalloys thereof is used to form the lattice structure, the advantageouseffects described in the foregoing can be achieved to a particularlycomprehensive extent. Irrespectively of the material selected toconstruct the lattice structure, the lattice structure can also beformed by material removal, thus not only by material application.

It proves particularly advantageous for the structural regions of thelattice structure to be arranged adjacent to sides of the waveguideridge and for the width of the waveguide ridge to be dimensioned suchthat base points of the sides are located in the peripheral region ofthe radiation emitted from the active zone of the laser layer. Thisensures that the amplification power of the laser is influenced to theleast extent possible by the lattice structure and in particular ensureseffective coupling between the laser radiation and the latticestructure.

In order to improve the electrical injection the metallic latticestructure is placed on a thin insulator layer (e.g. native or artificialoxide). The insulator layer should be chosen in such a way that therefractive index is closer to that of the semiconductor and that theinsulation is obtained a small thickness (typically a few nanometers).This layer is also used to suppress a potential penetration of thegrating material into the semiconductor material of the laser layers andtherefore serves as a barrier layer too.

For the optimization of the electrical injection surface and the effectsof the lattice structure, it also proves advantageous for the sides ofthe waveguide ridge to be arranged substantially at right angles to theplane in which the lattice structure extends, within the accuracyattainable by the manufacturing process.

In a process according to the invention, on the basis of a semiconductorsubstrate a complete semiconductor laser structure is produced in anepitaxial process with the subsequent formation of a waveguide ridge bysubjecting the semiconductor laser structure to a material removalprocess to form carrier surfaces arranged on both sides of the waveguideridge and subsequent application of a lattice structure to the carriersurfaces.

Irrespectively of the material selected to form the lattice structure,the lattice structure can also be produced by material removal, thus notonly by material application.

The processes according to the invention described in the foregoing thusfacilitate the production of functional laser diodes in a first processphase, thereby facilitating the precise checking and determination ofthe electrical and optical properties, thus for example determination ofthe individual amplification spectrum of the semiconductor wafer usedfor the laser fabrication. Only thereafter in a second process phase, bythe formation of lattice structures alongside the waveguide ridge withdefined parameters, are the originally multi-mode laser diodes convertedinto monomode DFB laser diodes with properties in each case defined as afunction of the parameters of the lattice structures.

In the event that the lattice structure is produced by the applicationof a lattice structure to the carrier surfaces, the use of alithographic process, in particular the use of an electron beamlithographic process with subsequent metallization of the lithographicstructure, proves particularly advantageous.

A variant of the production process which is particularly advantageousfrom the economic standpoint is possible if, for the production of aplurality of DFB laser diodes with different properties, a semiconductorlaser wafer is firstly produced by applying an epitaxial structure to asemiconductor substrate, whereupon the waveguide ridges associated withthe individual laser diodes are produced in the composite wafer byforming a strip-shaped waveguide structure which is arranged on thesurface of the semiconductor laser wafer and which comprises waveguideridges extending in parallel to one another and interlying carriersurfaces. Only thereafter is the semiconductor laser wafer divided upinto separate semiconductor laser chip units, whereby the propertiesassociated with the individual laser diodes are then precisely definedby the application or implantation of a lattice structure withcorresponding structural parameters on the surface of a selected numberof the laser diodes.

It is thus possible for the laser diodes which have been produced in thecomposite wafer and are already provided with the waveguide ridge to beused as basic laser diodes or “unfinished” laser diodes with definedelectrical and optical properties whereupon, from this reservoir ofidentically formed basic laser diodes, the required number of laserdiodes can then be selected and, by the application or implantation ofdefined lattice structures, the desired number of monomode DFB laserdiodes with precisely defined optical and electrical properties can beproduced substantially without rejects.

In the following the construction of an embodiment of a DFB laser diodeaccording to the invention and a possible process for the productionthereof will be explained in detail making reference to the drawing.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this disclosure. For a better understanding of the invention,its operating advantages and specific objects attained by its uses,reference is made to the accompanying drawings and descriptive matter inwhich preferred embodiments of the invention are illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 a is a view illustrating a stage in the production of a DFB laserdiode with a lateral lattice structure;

FIG. 1 b is a view illustrating a different stage in the production of aDFB laser diode with a lateral lattice structure;

FIG. 1 c is a view illustrating a different stage in the production of aDFB laser diode with a lateral lattice structure;

FIG. 2 is a scanning electron microscope image of a plan view of thelattice structure arranged on both sides of a waveguide;

FIG. 3 is a simplified perspective view of a laser diode illustratingthe active zone of a laser layer;

FIG. 4 is a diagram illustrating a possible amplification spectrum ofthe laser diode shown in FIG. 3;

FIG. 5 is a graphic representation of the dependence between theamplification spectrum or wavelength of the radiation emitted from thelaser diode and the lattice constant of the lattice structure;

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings in particular, FIG. 1 a is a simplifiedperspective view of a semiconductor laser or basic laser diode 10comprising a semiconductor substrate 11 and an epitaxial structure 12grown thereon. Part of the epitaxial structure 12 is formed by a laserlayer 13 based on a buffer and contact layer 31 and covered at the topby a covering layer 14.

The basic laser diode 10 shown in FIG. 1 a is of cuboid formation with aflat diode surface 16. Commencing from the basic laser diode 10illustrated in FIG. 1 a, the embodiment of a DFB laser diode 22according to the invention shown in FIG. 1 c is produced in twoessential process phases; as a transitional stage following theimplementation of a first process phase, FIG. 1 b shows a waveguidediode 17 in which the diode surface 16 has been subjected to a materialremoval process, such as for example a dry etching process, in order toobtain the illustrated stepped surface formation with a waveguide ridge15 extending in the longitudinal direction of the waveguide diode 17.The aforementioned material removal process gives rise to surfaces whichare formed on both sides of sides 18, 19 of the waveguide ridge 15 andwhich will be referred to in the following as carrier surfaces 20 and21, the carrier surfaces being covered by a thin insulating layer 26.

Commencing from the waveguide diode 17 illustrated in FIG. 1 b, theembodiment of a DFB laser diode 22 shown in FIG. 1 c is produced byforming a metallic lattice structure 23 with two structural regions 24and 25 in each case arranged in the carrier surfaces 20 and 21respectively by subjecting the carrier surfaces 20 and 21 to an electronbeam lithographic process and a following metallization process notdescribed in detail here. This second process phase results in the DFBlaser diode 22 illustrated in FIG. 1 c with the metallic latticestructure 23 arranged in the carrier surfaces 20 and 21 above the laserlayer 13. To be able to precisely define the position of the structuralregions 24 and 25 of the metallic lattice structure 23 arranged on bothsides of the waveguide ridge 15 in the epitaxial structure 12 relativeto the laser layer 13, the insulating layer 26, for example in the formof an etch-stop layer, is provided in the epitaxial structure above thelaser layer 13, which insulating layer 26 limits the depth, in theepitaxial structure 12, of a lithographic structure produced using anetching process and thereby defines the position of the metallic latticestructure 23 relative to the laser layer 13.

As shown by the electron microscope image of a plan view of the metalliclattice structure 23 schematically illustrated in FIG. 1 c, thestructural regions 24 and 25 constructed from lattice ridges 27, herearranged equidistantly from one another, extend up to the sides 18 and19 of the waveguide 15. The characteristic of the metallic latticestructure 23 is determined by the distance between the lattice ridges 27or the lattice constant d, the geometric configuration of the latticeridges 27, and the metal used for the metallic lattice structure 23.

FIG. 3 is a qualitative illustration of the active zone 28 of the laserlayer 13 in the form of an intensity distribution depicted in the outletcross-section of the laser diode 22. It will be apparent that, since theindividual lattice ridges 27 are connected as directly as possible tothe sides 18, 19, in particular in the region of base points 29, 30 ofthe sides 18, 19 a coupling is advantageously achieved between themetallic lattice structure 23 and the laser radiation in its peripheralzone.

FIG. 4 clarifies the filter effect obtained by means of the metalliclattice structure 23 in that, as can be seen from FIG. 4, subsidiarymodes of the laser radiation emitted from the active zone 28 areeffectively suppressed and substantially only the emission of one lasermode with a precisely defined wavelength is permitted.

FIG. 5 illustrates the effects of changes in the lattice constant d(FIG. 1 c) upon the wavelength. It will be apparent from FIG. 5 that bychanging the lattice constant d it is possible to achieve a highlyaccurate fine adjustment of the wavelength so that, commencing from apredetermined individual amplification spectrum of a basic laser diode10 illustrated by way of example in FIG. 1 a, by purposively selectingthe parameters of the lattice structure, thus for example here thelattice constant, a highly accurate setting of the wavelength can beobtained for the intended application of the laser diode in question.For example, by means of the choice of metal for the metallic latticestructure 23, the complex coupling between the laser radiation and thelattice structure, thus for example the absolute value and the relativeratio of the real—and imaginary parts of the refractive index—and thusthe component of refractive index coupling and absorption coupling—canbe set within wide limits. The so-called “duty factor” of the latticestructure, thus the ratio of the lattice ridge width to the latticeconstant, also constitutes a variable which can be optimized.

While specific embodiments of the invention have been shown anddescribed in detail to illustrate the application of the principles ofthe invention, it will be understood that the invention may be embodiedotherwise without departing from such principles.

1. A process for production of a semiconductor laser based on asemiconductor substrate with a laser layer arranged on saidsemiconductor substrate and a strip-shaped lattice/grating structure,the process comprising the steps of: producing a complete semiconductorlaser structure in an epitaxial process; forming a waveguide ridge bysubjecting said semiconductor laser structure to a material removalprocess to form carrier surfaces arranged on both sides of a waveguideridge; and applying the lattice/grating structure to said carriersurfaces.
 2. The process according to claim 1, wherein before applyingthe lattice/grating structure to said carrier surfaces an insulatinglayer is formed on the carrier surfaces.
 3. The process according toclaim 1, wherein said step of applying the lattice/grating structureincludes applying a metallic lattice/grating structure with alithographic process, said lithographic process being employed followedby metallization of said lithographic structure.
 4. The processaccording to claim 3, wherein a plurality of semiconductor lasers areproduced in a composite wafer in according to the steps comprising:producing a semiconductor laser wafer by application of an epitaxialstructure to a semiconductor substrate; forming the ridge-like waveguidestructure comprising waveguide ridges extending in parallel to oneanother on said surface of said semiconductor laser wafer; dividing saidsemiconductor laser wafer into individual semiconductor laser chipunits; forming one or more lattice/grating structures on saidsemiconductor laser chip units.